Plasma tuning means wherein the resonant frequency of a cavity resonator tracks the frequency of an ionizing control frequency



3,525,953 FREQUENCY OF S. L. H ALVERSON PLASMA TUNING MEANS WHEREIN THERESCNAN'? A CAVITY RESONATOR TRACKS THE FREQUENCY OF AN IONIZING CONTROLFREQUENCY 2 Sheets-Sheet l 7 0 6 9 1 7 2 N or d u w. A m

R %N wfibwmmzmw A N\ m m mmim l u N QM. u u Wm x KW \11 -\N k3 8c mmakmxi I; n F X X r @w mmQkQumk kwzak @w \T H I my 1Q mmmwduwk k2 Pfimm\sQmRmvfiE Suki Inventor jtemen L. Hair/e750 fitter/19v 5, 1970 s. lHALVERSON PLASMA TUNING MEANS WHEREIN THE RESONANT FREQUENCY OF A CAVITYRESONATOR TRACKS THE FREQUENCY OF AN IONIZING CONTROL FREQUENCY 2Sheets-8heet 2 Filed Nov. 7, 196'? 698 DETECTED FREQUENCY IN MHZ, ImMODE 5 xutwbommhb mtbo mwkmxdx ms [f2 uezziaf fez/e12 L. Ha [Verso/z ZrUnited States Patent US. Cl. 33373 Claims ABSTRACT OF THE DISCLOSURE Avariable-frequency signal generator is electrically coupled to aresonant cavity containing an ionized gas. The resonant frequency of thecavity may be changed by varying the frequency of the electricallycoupled signal.

CONTRACTUAL ORIGIN OF THE INVENTION The invention described herein wasmade in the course of, or under, a contract with the United StatesAtomlc Energy Commission.

BACKGROUND OF THE INVENTION This invention relates to resonant cavitiesand more particularly to means for varying the resonant frequency of acavity.

The resonant cavity is widely used in the electrical art as a resonantcircuit. The characteristics of a resonant cavity are similar to thoseof a conventional LC resonant circuit, with the resonant cavity having amuch higher efficiency than conventional circuits in the microwaveregion. It is often desirable to vary the frequency at which resonanceoccurs within a cavity. Present cavities accomplish this by changing thephysical size or shape of the cavity. It will be appreciated that, wherea cavity is in an inaccessible location, it is diflicult to change thecavity resonance by changing the physical shape of the cavity. Further,changing of the physical dimensions of a cavity is slow and oftendifficult even when accessible.

Accordingly, it is one object of the present invention to provide animproved means for changing the resonant frequency of a cavity.

It is another object of the present invention to provide means wherebythe resonant frequency of a cavity may be changed remote from thecavity.

Other objects of the present invention will become more apparent as thedetailed description proceeds.

In general, the present invention comprises a cavity in combination withan ionizing gas within the cavity. Means are provided for generating afrequency-variable signal which is electrically coupled to the cavity toeffect changes in the resonant frequency thereof.

BRIEF DESCRIPTION OF THE DRAWINGS Further understanding of the presentinvention may best be obtained from consideration of the accompanyingdrawings wherein:

FIG. 1 is a schematic diagram of an apparatus for the practice of thepresent invention.

FIG. 2 is a graphical representation of detected frequency vs.excitation frequency for a cavity operated according to the presentinvention.

FIG. 3 is a schematic diagram of an apparatus used to obtain the valuesplotted in FIG. 3.

In FIG. 1, a cavity has mounted therein cavity loop couplers 12 and 14which are respectively connected to a receiving antenna 16 and adetector 18. This is a typical structure for a conventional cavity in asignal-receiving system. For the present invention, to this structure isadded a cavity loop coupler 20 mounted within the cavity 10. A klystronoscillator 22 has its output connected via coaxial cable 24 to the loopcoupler 20 and hence the cavity 10. A wide-band double-stub tuner 26 isconnected to the coaxial cable 24 as shown. A vacuum pump 28 is coupledto the cavity 10 to permit partial evacuation of the interior thereof.

The apparatus of FIG. 1 operates to change the resonant frequency of thecavity 10 as follows. With the cavity partially evacuated by the vacuumpump 28, the klystron 22 is energized to deliver a signal ofpredetermined frequency to the cavity 10. The double-stub tuner 26 isadjusted to match the impedance of the klystron 22 and the impedance ofthe cavity 10 so that minimum power loss of signal is insured. The poweroutput of the klystron 22 is set so that it is suflicient to ionize theremaining gas within the interior of the cavity 10. With the remaininggas within the interior of the cavity 10 ionized, a change in thefrequency of the output signal from klystron 22 results in a change inthe resonant frequency of the cavity 10. Thus, the resonant frequency ofthe cavity 10 may be changed, whereby detection of selected frequenciesreceived by antenna 16 may be effected with detector 18.

It is to be noted that for the practice of the present invention theklystron 22 may be operated in one mode to tune the cavity 10 and effectdetection of frequencies in another mode by the detector 18. Forexample, the klystron 22 may be operated in the TM mode to tune thecavity 10, while signals in the TB mode are detected by detector 18.Using this mode of operation, electrical isolation between the cavityloop coupler 20 and the signalreceiving and detecting loop couplers 12and 14 is 40 db or better. Enhanced electrical isolation may be achievedby using selective band-pass filters in the receiving stage.

To further illustrate the resonant frequency tuning of the cavity 10 bychanging the output frequency of the klystron 22, reference is made toFIG. 2 wherein are shown graphical plots of excitation frequency outputfrom klystron 22 vs. detected frequencies by detector 18. To obtain thedata illustrated in FIG. 2, the apparatus of FIG. 3 was used. It will benoted that the apparatus of FIG. 3 is the same as in FIG. 1 except thatthe conventiona receiving antenna 16 has been replaced by a sweep signalgenerator 30. Further, the cavity 10 is shown lined with a bell jar 32which extends the length of the cavity 10 but is spaced with respect tothe sides thereof and is sealed to the cavity at one end as shown. Thebell jar was used to facilitate partial evacuation of the cavity 10 andis not a limitation on the present invention. The present invention willwork on cavities Which do not contain a bell jar.

The cavity 10 in FIG. 3 had a diameter of 33 cm. and a length of 42 cm.with a characteristic resonance of 686.234 megahertz. As stated, thebell jar extended the length of the cavity 10 and was spacedapproximately /2 inch from the interior walls thereof. The partialvacuum within the bell jar 32 was varied from .010 to .025 torr fordifferent measurements. The output power of the klystron 22 wasmaintained at a level of 24 watts and was sufficient to ionize theremaining air within the bell jar 32. The klystron 22 was operated inthe TE mode and the sweep generator 30 was operated in the TMOIO mode.

The resonant frequency of the cavity 10 was varied as hereinbeforedescribed for the apparatus of FIG. 1. To detect the resonant frequencyof the cavity, the sweep generator 30 was used. The curves 34, 36, 38and 40 of FIG. 2, respectively, represent results obtained at partialbell-jar vacuum values of .010, .015, .020 and .025 torr, respectively.Curves 34, 36, 38 and 40 of FIG. 3 clearly 3 show that, if theexcitation frequency from klystron 22 is changed, the resonant frequencyof the cavity changes. For example, when operating with a pressure of.025 torr, a change in the frequency of klystron 22 from 625 megahertzto 627 megahertz in the TE mode causes the resonant frequency of thecavity 10 to change from 696 megahertz to 698 megahertz in the TM mode.

It is to be understood that gases other than air may be used within theinterior of the cavity 10 to elfect the present invention, the onlylimitation on such gases being that they be capable of being ionized.Further, such gases may be at pressures other than those described. There duced pressures set forth were used to facilitate ionization of airand are not intended to form a limitation on the invention. It will beappreciated that the present invention readily lends itself to remotecontrol of the resonant frequency of a cavity and permits the adjustmentof the resonant frequency of a cavity more rapidly than accomplished bychanging the physical dimensions thereof, as heretofore taught in theart.

Persons skilled in the art will, of course, readily adapt the generalteachings of the invention to embodiments far different from theembodiments illustrated. Accordingly, the scope of the protectionatforded the invention should not be limited to the particularembodiment illustrated in the drawings and described above, but shouldbe determined only in accordance with the appended claims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. An apparatus for varying the resonant frequency of a cavitycomprising an ionized gas within said cavity, means for generating afrequency-variable signal, and means for electrically coupling saidgenerated signal to said cavity, the frequency of said generated signalbeing 3 determinative of the resonant frequency of said cavity.

2. The apparatus according to claim 1 wherein said electrical couplingmeans comprise a coaxial line connected to said signal-generating means,loop coupling means terminating said coaxial line to said cavity, andmeans for matching the impedance of said cavity to the impedance of saidsignal-generating means.

3. The apparatus according to claim 2 wherein said impedance-matchingmeans comprise a double-stub tuner electrically connected to saidcoaxial line to vary the impedance thereof.

4. In combination with a cavity containing a gas, means for partiallyevacuating said cavity, means for generating a frequency-variable signalat a power level to ionize gas remaining within said. partiallyevacuated cavity, and means for electrically coupling saidfrequency-variable signal to said partially evacuated cavity to vary theresonant frequency thereof responsive to the frequency'of said generatedsignal.

5. The apparatus of claim 4 wherein said coupling means comprise coaxialmeans connected to said signalgenerating means and loop-coupled to theinterior of said cavity, and means interconnected of said coaxial linemeans to match the impedance of said cavity and said signal-generatingmeans.

References Cited UNITED STATES PATENTS 2,659,028 11/1953 Kyhl 315-392,660,711 11/1953 Garbuny 333-17 3,348,169 10/1967 Tomeyasu 315-39 OTHERREFERENCES HERMAN KARL SAALBACH, Primary Examiner W. N. PUNTER,Assistant Examiner US. Cl. X.R.

